Create a web part zone that spans 2 columns?

I have a team site which has two web part columns, left column (70%) and right column (30%).  I'd like to keep those two columns and create another web part zone below them that spans both columns at 100%.  An example would be:

left column (70%) | Right column (30%)
bottom column (100%)

How can this be done?

Any help would be most appreciated.